The present invention relates to a drive axle assembly for a vehicle having multiple drive axles, and more particularly, to such an assembly which is capable of transmitting torque, selectively, to less than all of the drive axles.
The present invention is generally applicable to any vehicle drive system which includes multiple drive axles which are normally intended to rotate at about the same speed. However, the present invention is especially adapted for use with tandem axle assemblies and will be described in connection therewith.
The essence of a tandem device is its ability to drive more than one drive axle. This is typically accomplished by means of a power (or torque) divider, usually in the form of a differential in which one of the differential side gears transmits a certain percentage of the input torque to the first drive axle, while the other differential side gear transmits the remaining torque by means of an output shaft to a second drive axle or to a second tandem axle assembly which can then divide the remaining torque between second and third drive axles. A tandem axle assembly of the type well known in the art is illustrated in U.S. Pat. No. Re. 25,269 (originally U.S. Pat. No. 3,000,456), assigned to the assignee of the present invention, and which is incorporated herein by reference.
Tandem axle assemblies have become increasingly popular for use in the trucking industry, especially for vehicles subjected to relatively heavy loads, because of their ability to provide multiple pairs of driving wheels. However, it is a feature of tandem axle assemblies that torque is continuously being transmitted to both the adjacent (first) drive axle as well as the subsequent (second) drive axle, which is desirable under conditions of extremely heavy load or under unfavorable road conditions (mud, ice, etc.) Under such road conditions, a common problem with tandem axle assemblies has been the "out" which occurs when one or more of the drive wheels loses traction, causing the particular wheels as well as their associated drive mechanism all the way back to the differential to "spin out" to a higher rotational speed than the other driving elements of the system.
On many types of vehicles, especially those generally referred to by the designation "6 .times. 4", it may not be necessary to have torque continuously transmitted to both the rear drive axles, but instead, to transmit all of the driving torque to one of the drive axles under normal operating conditions (lightly loaded, dry highway), and have the ability to divide the torque among multiple drive axles in response to certain driving conditions. This is especially desirable in view of the manufacturing costs and complexity of conventional tandem axle assemblies utilizing continuous torque dividing mechanisms such as differential gear sets.
A drive axle assembly which transmits torque between first and second drive axles only when really necessary would have the additional advantage of prolonging the operating life of all of the driving components and gearing associated with the second drive axle, as they would be in driving operation only infrequently, and for relatively short periods of time.